1. Field of the Invention
The present invention relates generally to electronic devices, and, more specifically, to resistive memory devices.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Generally, resistive memory devices manipulate and sense the resistance of a memory element. Typically, the memory element is capable of assuming two or more resistance states, e.g., high resistance and low resistance. To store data, the memory element is driven to one of the resistive states. Then, to read the data, the resistance of the memory element is sensed. For example, in a binary system, a low resistance may correspond to a value of zero, and a high resistance may correspond to a value of one. Binary data stored in a resistive memory element is typically read by sensing if the resistance of the element is greater than or less than a threshold resistance. During a read operation, the sensed resistance is compared to the threshold resistance, and the resistive memory device outputs a one or a zero based on the comparison. Thus, the resistance of the resistive memory element indicates the content of stored data.
The resistance of the memory element is typically measured indirectly. Resistive memory devices often read data by measuring a transient response of a sensing circuit that includes the memory element. Generally, a stimulus, such as a sudden change in voltage, is applied to the sensing circuit, and, after a waiting period, a parameter, such as a voltage at some node, is sensed. Depending on the electrical state of the sensing circuit after the waiting period, the resistive memory device may output a zero or a one. For example, in some sensing circuits, a low-resistance state may result in a rapid increase in the voltage of the sensed node, and a high-resistance state may result in a slow increase in the voltage. In these sensing circuits, the voltage sensed immediately after the waiting period indicates the resistance of the memory element and the value of the stored data, e.g., a low voltage indicates a slow response and a high resistance, and a high voltage indicates a fast response and a low resistance. That is, the resistance of the memory element affects the speed of the sensing circuit, and the speed of the sensing circuit indicates the value of the stored data.
Unfortunately, read operations may take a long time. Memory elements storing a given logic value may have a large distribution of resistances. For instance, some memory elements storing a zero or a one may have a resistance that is close to the threshold resistance. In many sensing circuits, small differences in resistance take a relatively long time to discernibly affect the sensed parameter during read operations. Circuit designers often design for a worst-case scenario, so the waiting periods are typically relatively long to discern resistances that are close to the threshold resistance. In other words, circuit designers may increase the waiting period to measure accurate resistance states, as some resistances may be close to the threshold resistance. Consequently, read operations may be relatively slow in some resistive memory devices.